Rspec: test number of rows in CSV output

In Rspec how do I test the number of rows that are in a CSV file?

I have created a CSV file in a mutation. I am able to test the output of the CSV by checking if certain text is included. However, I would like to be able to test the number of rows that are in the output.

Please find an example of what I have tested so far below which passes:

describe "#execute" do

it "creates a CSV for the specified content" do
outcome = mutation.run(mutation_params)